It is known that a latent electrostatic image can be developed with toner particles dispersed in a carrier liquid, generally an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid toners comprise a thermoplastic resin and nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0, and a high vapor pressure. The toner particles are less than 10 .mu.m average by area size. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
Since the formation of proper images depends on the differences of the charge between the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound and preferably adjuvants, e.g., polyhydroxy compounds, polybutylene succinimide, an aromatic hydrocarbon, etc. to the liquid developer comprising the thermoplastic resin, nonpolar liquid, and preferably a colorant. Such liquid developers provide images of good resolution, but it has been found that charging and image quality are particularly pigment dependent. Some formulations suffer from low or poorly controlled particle mobility resulting in poor image quality manifested by low resolution, poor solid area coverage, and/or image squash. Furthermore, some formulations result in wrong sign (negative) developers. In order to overcome such problems much research effort has been expended to develop new type charge directors and/or charging adjuvant for electrostatic liquid developers.
It has been found that the above disadvantages can be overcome and improved positive developers prepared as described below containing a nonpolar liquid, a thermoplastic resin, a charge director compound mixture described below, and preferably a colorant. The improved positive electrostatic liquid developer charged with the charge director compound mixture when used to develop an electrostatic image results in image quality, squash, and solid area coverage comparable to other known charge directors with the additional advantage that for a given liquid developer the charge director compound mixture can be controlled to optimize liquid developer performance.